[FIRRTL] Draft PR of changes to Grand Central taps

[jackkoenig]

This is my work-in-progress for changing the Grand Central taps and memtaps annotations to no longer rely on using FIRRTL extmodules but instead just annotate the source of the XMR and the sink (called wireName at the moment but it would probably be better to just call this sink).

This work is incomplete, I'm PRing it to make it easier to collaborate with @prithayan since we may just ditch the old implementation of Grand Central taps in favor of the new FIRRTL XMR support that @prithayan has been working on.